1. Technical Field
The present invention relates to a rotation sensor, and more particularly, to a contact type automotive rotation sensor.
2. Related Art
In recent years, with increased attention to the global environmental problem, reduction of automotive body weight have been made as a measure to reduce automotive emission. By way of example, a conventionally mainstream hydraulically-driven power steering apparatus has been replaced by a power steering apparatus of electrically-driven type which can realize automotive weight reduction by eliminating a hydraulic pump.
However, as compared to a hydraulically-operated power steering apparatus capable of generating a proper assist force solely by use of a mechanical mechanism, an electrically-driven power steering apparatus requires one or more rotation sensors for detecting the steering angle and/or steering torque to enable a computer to calculate an assist force based on steering angle and steering torque.
Rotation sensors for an electrically-driven power steering apparatus are classified into a non-contact type sensor that detects the steering angle and/or torque based on a change in magnetic field caused by steering operation and a contact type sensor that makes detection on the basis of a voltage change caused by steering operation and detected through a contact and an electrical resistor which are disposed for relative sliding motion.
A contact type rotation sensor includes a ring-shaped electrical resistor formed on a surface of a disk-shaped base plate that is rotatable with rotation of a steering handle, and a contact disposed in sliding contact with the electrical resistor. In the rotation sensor, a predetermined voltage is applied across two reference points of the electrical resistor, and a voltage signal is taken out from the contact.
As the steering handle rotates, relative sliding motion occurs between the electrical resistor and the contact, and the voltage signal taken out from the contact varies depending on the distance of relative sliding motion. Thus, the steering angle and/or steering torque can be measured based on the voltage signal.
However, the conventional contact-type rotational sensor has a drawback that sliding sound is produced during the relative sliding motion, giving discomfort to a person.
Furthermore, most components of the rotation sensor are usually fabricated by resin molding at low costs, with an allowable manufacturing error. Thus, the distance between the contact and the base plate can vary normally about xc2x11 mm though a variation in distance differs depending on the allowable manufacturing error. As a result, there occurs a variation in contact force between the contact and the electrical resistor formed on the base plate to render a connection state therebetween unstable, resulting in poor durability of the rotation sensor.
An object of the present invention is to provide a contact type rotation sensor capable of suppressing sliding sound and realizing improved durability by ensuring a stable contact state between a contact and an electrical resistor.
The present inventors conducted measurements of sliding sound generated during relative sliding motion between a contact and an electrical resistor of a rotation sensor, and analyzed measurement results with use of a computer, etc. to find the following facts. Specifically, relative sliding motion between a contact and an electrical resistor produces friction that serves as energy source of torsional vibration of the contact. The torsional vibration causes sliding sound. In a case where the contact is formed into a shape particularly liable to make vibration, torsional vibration promotes natural vibration of the contact, producing extremely loud sound at natural frequencies, to thereby give great discomfort to a person. Thus, the present inventors recognized the necessity of finding a shape which makes a contact hard to vibrate, and made concentrated experiences to find such a shape.
Furthermore, the present inventors considered that a proper contact force must be produced between a contact and an electrical resistor in order to establish a stable contact state therebetween to improve the durability of a rotation sensor, and made concentrated experiences based on the recognition that a contact is required to have a proper spring constant to produce a proper contact force.
A rotation sensor according to the present invention, created based on results of the experiences, comprises: a base plate that is rotatable; an electrical resistor formed on a surface of the base plate so as to extend in a rotating direction of the base plate; a holder disposed near the base plate; and a contact member fixed at its one end portion to the holder and provided at another end portion with a contact, the contact member having an arm portion thereof extending between these end portions. Rotation of the base plate causes relative sliding motion between the contact and the electrical resistor, with the contact being in electrical contact with the electrical resistor. The arm portion has asymmetric widthwise weight distribution about a longitudinal axis of the contact member. Information on the rotation of the base plate, produced during the relative sliding motion between the contact and the electrical resistor, is taken out from the contact member in a form of electrical signal.
According to the rotation sensor of the present invention, the contact member is provided with the arm portion that has asymmetric widthwise weight distribution about the longitudinal axis of the contact member, and accordingly torsional vibration of the contact member is suppressed that is generated during relative sliding motion between the electrical resistor and the contact of the contact member, and such torsional vibration may occur not only at natural frequencies but also at dispersed frequencies other than natural frequencies, whereby the sound pressure of sliding sound caused by the relative sliding motion can be reduced.